1. Field of the Invention
Embodiments of the present disclosure relate to the field of optical communication systems. More particularly, the present disclosure relates to a method and system for fault recovery in trunk and branch optical add/drop multiplexing (OADM) networks.
2. Discussion of Related Art
Undersea fiber optic communication systems may include a main trunk path extending between land-based cable stations and one or more branch segments connected thereto. The main trunk is defined by an undersea cable having a plurality of optical fibers therein and one or more repeaters or optical amplifiers disposed along the trunk path used to amplify optical signals transmitted between the cable stations. Each cable station includes terminal equipment used to transmit and receive these optical signals along the main trunk path. The one or more branch segments are coupled to the trunk through a branching unit (BU) at one end and to a branch segment cable station at another end. These systems are referred to as trunk and branch networks. Trunk cable stations may be used to carry information signals through the backbone of the network while the branch segments may be used to transmit or receive traffic between the trunk paths to the branch cable stations. The optical signals transmitted between the trunk and branch cable stations are typically dense wavelength-division multiplexed (DWDM) signals in which a plurality of optical channels, each at a respective wavelength, are multiplexed together.
Historically long haul undersea trunk-and-branch networks have been used to provide connectivity between cable stations by using dedicated fiber pairs. A more recent architecture, employs optical add/drop multiplexers (OADMs) to provide a more flexible distribution of transmission connectivity in comparison to technologies using fiber pairs. In general, an OADM node is used to add and/or drop channels within a DWDM optical signal between the trunk and branch segments. The advantages of utilizing OADMs, in part, stems from the ability to share capacity of dedicated fiber pairs among multiple network branches.
In typical optical trunk and branch network configurations, not all the system bandwidth is utilized at initial deployment. Consequently, the initially deployed data channels may experience higher optical power which may cause system performance degradation. In these cases, initial loading equipment (ILE) is employed to transmit non-payload carrying signals within the system bandwidth or transmission spectrum between cable stations. In other words, the ILE may be used to fill up un-used capacity of the network before most or all system bandwidth are deployed as payload channels.
ILE may transmit and receive discrete tones which are referred to as discrete tone initial loading equipment (DT-ILE). Subsequently, as each payload data channel is being added into the network, the ILE is replaced by optical terminal equipment which transmit/receive payload channels within the network. Thus, depending on the amount of system bandwidth used for data transmission, the ILE may consume more or less portions of repeater power in the system until all system bandwidth are utilized by data traffic in the network. In the case of DT-ILE, this power consumption may take place over particular frequencies or wavelengths that may border on the high and/or low range of channel frequencies used for payload transmission. As a result, the insertion of the ILE signals may also serve the purpose of power management in the optical network to ensure the installed data channels are at a preferred power level.
In OADM trunk and branch networks, optical power management remains a challenge, especially when there a cable fault occurs. Cable faults that interrupt traffic, such as cable cuts, can cause transmission loss between cable stations. This incidence can lead to severe optical power changes with remaining optical channels in the network. FIG. 1 illustrates a conventional and simplified OADM trunk and branch network 10 including trunk cable stations or terminals 12 and 14 connected via trunk path 16. Branching units 18 and 20 couple branch cable stations or branch terminals 30 and 32 to trunk path 16 through respective branch segments 34 and 36. Each of the branching units 18 and 20 include OADM nodes used to add/drop channels propagating between trunk path 16 to branch segments 34 and 36. Trunk path 16 is defined by an optical cable having a plurality of optical fiber pairs, optical amplifiers 16a, 16a1, 16b, 16b1, 16c, 16c1 disposed along the optical cable as well as other optical/electrical equipment used to transmit optical signals along the trunk path 16 from between terminals 12 and 14. Typically, the optical signals or “through traffic” travel along trunk 16 between terminals 12 and 14, whereas signals destined for branch terminals 30 and 32 are added/dropped from the trunk 16 using OADM nodes in branching units 18 and 20 respectively. For each fiber pair along trunk 16 there are two corresponding fiber pairs within each branch segment 34, 36 in order to provide transmission capacity in both directions to/from branching unit 18 and branch terminal 30 as well as in both directions to/from branching unit 20 and branch terminal 32 thereby supporting connectivity between all terminals 12, 14, 30 and 32.
If system 10 is fully loaded and a cable cut 40 occurs along branch 34, the cut may result in an optical power surge associated with the channels between terminals 12 and 14 in order to maintain the level of optical power over the system within a preset range. FIG. 2 depicts possible optical power spectra that may be detected at terminal 12 during system operation before a cut (50) and after a cut (52). In this example, data channels (represented by portion 54) are allocated for the traffic between station 30 and 14. Data channels (represented by portion 56) are allocated for the traffic between station 12 and 14. The signal power level 50 before a cut 40 may correspond to a level in which optical signals are properly transmitted along trunk path 16 without error or with an error rate within acceptable limits. When the cut 40 occurs, data channels 54 will be out of service because of the discontinuity of the fiber path for data channels 54. Meanwhile, data channels 56 remain propagating in the trunk path 16 with an increased power level. However, if after cut 40 occurs, the optical signal power level 52 for data channels 56 exceed a level at which optical signals can be properly transmitted between terminals 12 and 14, disruption of payload traffic in data channels 56 in system 10 may result in trunk path 16. In view of the above it will be apparent that a need exists to remedy undersea OADM networks when a cable fault occurs, referred as OADM fault recovery.